1. Field of the Invention
The present invention relates to a magnetic screw device to convert rotary motion to linear motion by combination of a male magnetic screw and a female magnetic screw each of which is spirally magnetized on the outside surface, and more particularly to a magnetic screw device with high strength, high precision and high degree of free designing, in which a male magnetic screw or a female magnetic screw is formed from cylindrical magnets which are relatively short in length and spirally magnetized, and a method of manufacturing the magnetic screw device.
Furthermore, the present invention is concerned with a magnetic screw conveying device capable of keeping the balance of magnetic force between a male and female magnetic screws to always produce stable motion by covering a male magnetic screw or a female magnetic screw with a protecting tube.
The present invention also relates to a magnetic screw conveying device capable of driving a male and female magnetic screws while holding them constantly in noncontact each other by constructing male magnetic screw of divided blocks fitted on a rod member and by supporting rotatably the rod member having the divided male magnetic screw connected on a support stand.
2. Description of Related Art
There have been proposed various types of magnetic screw devices in which a movable member spirally magnetized on the outer surface is contained in a cylindrical housing spirally magnetized on the inner surface so as to correspond to spiral magnetization on the outer surface of the movable member, so that the rotating of the movable member or the housing can cause a linear motion of the movable member.
These magnetic screw devices have some advantages in that (1) they leave an excellent safety at the time of collision of the movable member because the movable member moves while being held in noncontact with the housing and each spirally magnetized portion only falls out of step even when the movable member had a collision with the housing; (2) the movable member being held in noncontact with the housing can reduce mechanical friction, thus preventing the occurrence of abrasion of members, dust powder, loss of transmission, and noise; (3) there is less backlash; (4) it enables remarkably free designing of a stroke, and (5) a driven section can be blocked from vibration caused by a power unit. Those advantages are not present in common mechanical conveying devices.
Accordingly, various applications of the magnetic screw to many purposes have been considered; for example, a general conveying device and an automatic door utilizing the safety at the time of collision, a conveying device used in a clean room by utilizing the advantage of no occurrence of abrasion dust powder due to the construction with no contacting portion, a conveying device for a vacuum equipment by utilizing the advantage of a noncontact construction needing no lubricating oil and enabling the use under a dry condition, a conveying device equipped in office automation equipment which is quiet due to noncontact construction, and a focusing device adapted for optical apparatus by blocking vibration caused by a power unit.
Conventional magnetic screw devices having the excellent characteristics mentioned above, however, have the following problems at the same time. Due to the disadvantages, consequently, actual production applications as discussed above have not been realized to a significant extent.
As a first disadvantage, a magnetic screw using a movable member and a housing both which are spirally magnetized is expensive because magnetization in a spiral form in a magnetic body is not easy and needs a complex process. Specifically, manufacturing such a lengthy screw is difficult and costly.
It is also difficult to manufacture magnetic screws capable of withstanding large load, because magnetic flux density does not raise easily, for instance, when a movable member is formed of a cylindrical magnetic material and magnetized its outer peripheral surface. It is especially difficult to manufacture a magnetic screw with high magnetic flux density in the case of magnetizing a long magnetic material all at once. To resolve this problem, the use of a large-sized magnetizing apparatus is required. This is not practicable.
In addition, it is difficult to obtain mechanical strength required in various devices, because when a movable member and a housing each having a long length are formed by pressing and burning processes, in particular, their center portions often become thin by the pressing process, tending to cause defects such as cracks and the like in the burning process. Many ferritic materials and rare earth materials being excellent magnetic material are brittle themselves, so that a magnetic screw made of such materials can not have sufficient mechanical strength. The existence of defects in the movable member and a housing will cause an increase of magnetic resistance inside of the magnet, resulting in a reduction of magnetic flux density accordingly.
To prevent the above problems, a method of winding spirally a magnetic band on a cylindrical member has been proposed in Japanese Patent Application laid-open No. 1-126465. This method, however, has problems that the magnetic band itself has only weak magnetic force and can not provide much accuracy in position due to differences in winding.
For conventional magnetic screw conveying devices described above, protecting the surface of a magnet by painting has been used because magnetic material is generally brittle.
However, the above conventional screw conveying devices in which the surface of magnet is protected by painting and the like have the following problems.
The magnetic material being generally brittle, if foreign substances bit and catch in a clearance between a male and female magnetic screws, a part of the male and female magnetic screws is likely to chip easily. If water and oil come into the clearance, rust is produced in pinholes and other parts of the painted surface of the magnet, resulting in a deterioration in the performance of the magnet.
Chipping a part of male and female magnetic screws and rusting in pinholes cause damage to design balance of magnetic force, resulting in the irregular conveyance of the magnetic screw conveying device. Thus, the magnetic screw conveying device can not make a stabilized conveying motion.
A conventional example of magnetic screw conveying devices disclosed in Japanese Patent Application No. 1-209222 will be described in reference with FIG. 19. A shaft 151 is rotatably supported on a couple of ball bearings 152 fixedly mounted on a frame. On the outer surface of the shaft 151, magnetized bands 153 with a north pole N and a south pole S are wound alternately and arranged in a spiral form, thus forming a male magnetic screw 171. On an end of the shaft 151 supported on the ball bearings 152, a pulley 154 is fixedly attached. Between the pulley 154 and another pulley 157 of a motor 156, a belt 155 is stretched.
Slide table 158, as its sectional view is shown in FIG. 19, is constructed so as to surround the male magnetic screw 171 and a guide bar 161 for preventing the slide table 158 from rotating during the sliding. On the inner surface of a cylindrical hollow 159 of the slide table 158 are provided magnet bands 160 with a north pole N and a south pole S wound alternately and arranged in a spiral form, thus forming a female magnetic screw 172.
In the cylindrical hollow 159, the male magnetic screw 171 is disposed with a clearance space indicated by "a" in the drawing so that the magnets 153 and 160 are not contact each other.
In the magnetic screw conveying device constructed as above, the rotation of the shaft 151 driven by the motor 156 produces magnetic force between the male and female magnetic screws 171 and 172, that is, the magnet 153 wound on the shaft 151 and the magnet 160 attached on the slide table 158. At this time, as the shaft 151 rotates, the slide table 158 moves linearly along the guide bar 161. In the meantime, if the rotation of motor 156 is reversed, opposite magnetic force works between both magnets 153 and 160, enabling the slide table 158 to move back.
Actually, those magnetic screw conveying devices proposed as above have not been readily used practically. The cause is in that if the shaft 151 is formed with a long length, it will be bent (see FIG. 20), making it impossible to move the slide table 158 without contacting the male magnetic screw 171.
Magnetic screw conveying devices can be used for conveying parts in factories and shipping vegetables in markets, for example, and there it needs linear movement in a long distance.
However, the shaft 151 is bent by the weight of the shaft 151 itself or by the attracting force produced between a pole N and a pole S of the male and female magnetic screws 171 and 172 during the operation. Specifically, in the case of the female magnetic screw 172 not being formed of an entire cylinder, for example, a semi-cylinder, the attracting force partially works on the shaft 151, so that the shaft 151 will be easily bent.
Accordingly, the male and female magnetic screws 171 and 172 are actually to work without contacting each other, but the female magnetic screw 172 tends to contact the male magnetic screw 171 at the bent portion thereof. In conventional magnetic screws, the space "a" between both magnetic screws 171 and 172 is set at about 0.5 mm, for example. In the case where a semi-cylindrical female magnetic screw is used, the shaft 151 having a radius of 15 mm will be bent by the attracting force when the length becomes 500 mm, causing the contacting of both magnetic screws.
Contacting of the male and female magnetic screws 171 and 172 can not produce any effect of non-contact movement which is a characteristic of a magnetic screw conveying device using magnetic screws. Namely, when the male and female magnetic screws 171 and 172 contact each other, it is impossible to obtain various advantages of a magnetic screw conveying device, not realized in mechanical conveying devices. As mentioned above, these include it is excellent in safety at the time of collision of the movable member because the movable member moves without contacting the housing and each spirally magnetized portion only falls out of step even when the movable member had a collision with the housing; the construction having no contacted portions can reduce mechanical friction, thus preventing the occurrence of abrasion of members, abrasion dust powder, loss of transmission, and noise; less backlash; remarkable free designing of a stroke; and a driven section can be blocked from vibration caused by a power unit.